But it's really a bit of a bogon, IMO, to think it means you have to be joyous all the time; fun is the result of happiness due to play (and other sources of enjoyment)--but that play can have unpleasant aspects to it as well as pleasant ones, and as long as the aggregate is enjoyable, its fun.

I mean, even outside of BDSM (though it's obviously relevant), horror movies, spicy food, and even romantic tear-jerkers, etc can have unpleasant bits or even wholes; games (even solitaire) can be frustrating or just horrifying (as the above mentioned one is). But the whole point, usually, is that the aggregate experience is, well, fun. You're not doing it for self-improvement or appearance's sake or cash and prizes, so at least in theory, there's an expectation of fun -- and if you do it again and again, one hopes it even delivers.

And you can add most sports to your list of complex hobbies. It's obviously true of training (though people tend to make training more painful than is safe or useful), but it also applies to spectators who'd rather stay attached to a team when it's losing than give up on it.